The present invention is directed to a piston ring arrangement for an internal combustion engine with one cylinder and a piston including a piston ring guided in the cylinder with the cylinder, the piston and the piston ring each formed of ceramic material.
In conventional internal combustion engines piston rings are employed for the well known task of sealing the combustion space from the crankcase, for aiding the heat transmission from the piston to the cylinder wall and for regulating the oil consumption of the engine. Accordingly, the piston rings bear at their outer periphery tightly against the cylinder wall with one surface or side bearing against a corresponding surface in the piston groove. This radial arrangement is effected by the spring force in the piston ring itself which is assisted by the gas pressure acting behind the piston ring. The axial contact of the piston ring is determined by the gas pressure and the acceleration forces and, therefore, alternates between the upper and lower ring sides.
To effect the various tasks provided by the piston rings, it is usual to employ separate piston rings as so-called compression rings and so-called oil scraper or control rings. These rings differ in cross-section. With the warping of the cylinder in present day lightweight construction engines, there has been a need for particularly flexible oil scraper rings. Such flexible oil scraper rings generally have a small crosssection and thus a low resistance moment and are tensioned by an additional spring located behind them with the spring supported in the bottom of the piston groove, note Bussien, "Handbook of Automotive Technology", Vol. 1, 18th Edition, 1965, page 258.
In engines where the cylinders and pistons are formed of ceramic materials, the tasks of regulating the oil consumption of such engines and of helping the heat dissipation from the piston to the cylinder wall are no longer applicable in view of the operating characteristics of ceramic material, in such engines merely the combustion space must be sealed off to achieve the desired compression ratio. Accordingly, oil scraper rings are no longer needed whereby only so-called compression rings must be used.
Since in such ceramic engines all parts in sliding contact with one another must be formed of the same material, the piston ring must also be made of a ceramic material. Ceramic piston rings, however, have a very low elasticity, because ceramics are considerably more brittle than the metals normally used for piston rings.
Therefore, such pistons must be guided as rectilinearly as possible along the piston stroke so that only so-called slider cranks are suitable such as shown and described in German Patentsschrift 920 758. Moreover, the ceramic piston is formed in two parts for insertion of the piston ring into the piston ring groove, since the elasticity of such a piston ring is insufficient to slide it over the piston skirt. Such a piston is formed of one piston part making up the piston skirt and another piston part forming the piston bottom and these parts must have oppositely facing surfaces which form an intermediate annular groove of rectangular cross-section open to the outside. In addition, the two piston parts must include an aligned concentric ring for the location of a sleeve which engages into a T-shaped abutment penetrated by concentric counterbores for connecting the piston to the piston rod. The number of components increases the production costs and complicates assembly and maintenance. The piston ring itself must be slit for affording a certain degree of elasticity required for the contact with the inside peripheral surface of the cylinder. It has been found appropriate to design the slit as a groove with a pin guided into it at right angles to prevent twisting of the piston ring in two-stroke engines.
Due to such a design, the piston ring is relatively expensive to fabricate and is very susceptible to fracture during assembly because of its brittle character.